THE AIM OF THIS PROJECT IS TO ACHIEVE DETAILED AND ACCURATE EXPERIMENTAL DATA ON THE STRUCTURE AND THE BEHAVIOUR OF TRANSITION PHASES ARISING FROM CO2 MISCIBLE FLOODING OF OIL RESERVOIRS, SO AS TO IMPROVE SWEEPOUT EFFICIENCY PREDICTIONS.
Research was carried out into the characterization of transition zones including a study of transport phenomena in order to acquire informations about the mechanism for the stabilization of flood fronts near the minimum miscribility pressure (MMP).
The evaluation of the relative permeabilities and viscosities were based on studies of the relative permeability governed by the phase behaviour within the transition zone.
Extensive pressure volume temperature studies on a live oil and carbon dioxide system were carried out and to evaluate partition coefficients of components from different classes of compounds analytical methods were developed and applied.
New experimental equipment for steady and unsteady state experiments was developed measurements using steady state processes. The flow was generated by displacement pumps driven by stepmotors. The back pressure regulator maintained the pressure within the hydraulic system and a set of pumps allowed injection of 2 coexisting phases into a reservoir model. An injection system was installed at the inlet of the core holder to study transport phenomena of characteristic oil compounds. The flood device for displacement experiments corresponded to conventional set ups for carbon dioxide flood experiments.
Flow experiment were carried out under steady state conditions and carbon dioxide flood experiments were performed to study the displacement behaviour below and above MMP.
The displacement behaviour of carbon dioxide and oil systems was not only governed by thermodynamical properties of the related phases, but also due to the transport behaviour of compounds involved. In contradiction to the results of the theory of the multicomponents chromatography, not including dispersion, the dispersion resulted in a continuous change of the concentration of all components. This was valid below and above MMP. The prevention of the separation of methane, ethane and carbon dioxide resulted in an elongation of the compositi on path within the homogeneous region of the transition zone. The crossover into the large methane and oil miscibility gap was avoided, which would lead to an unstable immis
cible gas drive at high viscosity differences and density differences. The miscibility gap was entered at high carbon dioxide concentrations resulting in minor interfacial tension in front of the transition zone.
BESIDE THERMAL METHODS, MISCIBLE FLOODING IS A MOST CONVENIENT METHOD TO ATTAIN ENHANCED OIL RECOVERY. ACTUAL EFFICIENCIES HOWEVER APPEAR TO BE OFTEN MUCH POORER THAN FROM LABORATORY SIMULATIONS BASED MAINLY ON ASTM ANALYTICAL DATA.
THE INSTABILITIES OF THE FLOOD FRONT ARE ASSUMED TO BE RESPONSIBLE FOR THIS; THESE, IN TURN, DEPEND UPON HOMOGENEITIES OF SITES, GRAVITY EFFECTS, LARGE DIFFERENCES IN VISCOSITY AND DENSITY BETWEEN CRUDE OIL AND FLOODING AGENT (I.E. CO2), VARIATION OF THE COMPOSITON OF THE TRANSITION PHASE OR ZONE (WHICH IS BUILT UP BY THE DRIVING MEDIA) AS A RESULT OF SELECTIVE OIL EXTRACTION (ESPECIALLY OF LIGHT N, S, O -COMPOUNDS)
UNDER UNSTABLE CONDITIONS, THE SIMULATED COMPOSITION PASSES THROUGH THE TWO-PHASE REGION. THE SWEEPOUT EFFICIENCY DEPENDS ON THE PHYSICAL AND CHEMICAL PROPERTIES OF THE TRANSITION PHASES. COLLECTING DETAILED AND ACCURATE EXPERIMENTAL DATA CONCERNING THEIR CHARACTERIZATION AS WELL AS THEIR FLOW BEHAVIOUR, EVEN IN POROUS MEDIA, IS THEREFORE A NEED FOR MORE RELIABLE SWEEPOUT EFFICIENCY PREDICTIONS. ON REAL SYSTEMS, CONSISTING OF CO2 AND TWO LIGHT TO MEDIUM OILS WITH VARIABLE INTERMEDIATE CONTENT, AT RESERVOIR TEMPERATURES AND PRESSURES BOTH BELOW AND ABOVE THE MINIMUM MISCIBILITY PRESSURE (MMP), THE FOLLOWING MEASUREMENTS ARE INTENTED TO BE CARRIED OUT IN HOMOGENEOUS AND HETEROGENEOUS PHASE REGIONS.
A) PVT-MEASUREMENTS ON EQUILIBRIUM PHASES (A MINIMUM OF 25 POINTS), DENSITY, VISCOSITY, PHASE VOLUMES, PARTITION COEFFICIENTS, INTERFACIAL TENSION. A PVT APPARATUS FOR WORK UP TO 150 CELSIUS DEGREES AND 100 MPA WILL BE USED. HPLC, GPC, GLC-MS TECHNIQUES WILL BE USED FOR COMPOSITION DETERMINATIONS.
B) STEADY AND UNSTEADY-STATE FLOOD EXPERIMENTS: RELATIVE PERMEABILITY (WHICH ALLOWS MULTIPHASE VOLUME STREAMS IN POROUS MEDIA TO BE DESCRIBED) AND DISPERSION OF MARKER COMPOUNDS (RELATED TO THE MASS TRANSFER BEHAVIOUR BETWEEN COEXISTING PHASES). EQUIPMENT WITH CORE DEVICE (10-20 MM IN DIAMETER, 20-100 CM IN LENGTH) AND SLIM TUBE (6-10 MM IN DIAMETER, 2-10 M IN LENGTH) AS RESERVOIR MODELS WILL BE USED. FOR UNSTEADY EXPERIMENTS THE CORE DEVICE WILL BE 10-90 MM IN DIAMETER.
C) MEASUREMENTS WITHIN THE TRANSITION ZONE DURING DISPLACEMENT: CONCENTRATION PROFILE AND LOCAL CONCENTRATION, DENSITY AND VISCOSITY. SPECIAL EQUIPMENT FOR ON-LINE DETERMINATIONS WILL BE DEVELOPED FOR THIS PURPOSE. SPECIAL ATTENTION WILL BE PAID TO THE PARTITION COEFFICIENTS OF HETEROATOM-CONTAINING OIL COMPONENTS. THE LOCAL CONCENTRATION OF THESE COMPOUNDS DURING DISPLACEMENT TESTS IS EXPECTED TO GIVE VALUABLE INFORMATION ON THE STRUCTURE AND STABILITY OF DISPLACEMENT FRONTS.